Sex and the Single Cell: Biologists Take a Fresh Look at "Asexual" Amoebas

Much of what we know about sex, or think we know, stems from the animal kingdom. No surprise there—we're animals and the nuances of the genetic tango are easier to study in organisms larger than infinitesimal blobs.

Trouble is, animal sex is specialized to the point of distraction. Most researchers have learned to avoid seeking universal sexual truths by examining animals' twig on the tree of life, but some still rely heavily on single animal models whereas others hawk dated taxonomic ideas without realizing it, says protistologist Frederick Spiegel of the University of Arkansas in Fayetteville.

"Huge numbers of trees are killed over the origin and function of sex, but some people writing and teaching this material still have animal sex in the back of their minds. It's biased, and it's backwards," says Spiegel, author of a commentary on sex published online May 10 in Proceedings of the Royal Society B.

Enter the amoeba: a collection of blobby, typically asexual microorganisms that taxonomists have historically swept under the rug as evolutionary oddities. If biologists want to understand sex's universal features, especially its benefits and costs, Spiegel argues there's no better critter to start with.

"Sex is one of the most primitive characteristics of all eukaryotic life," he says. "There are only a few eukaryotic lineages where we've never seen sex, and they're all amoebae. With these asexual organisms, we can compare and ask some truly synthetic questions about sex.”

Amoebas are single-celled blobs that house their DNA in nuclei, just like all of their eukaryotic relatives (humans included). Although some amoebas presumably cannot have sex and divide by mitosis, others are among the eukaryotes that can have sex—a process that can most simply be defined as ripping a genome in half and later recombining it. The practice fuels diversity by juggling genes and ultimately helps lineages weather catastrophic change over generations as natural selection acts on them.

The historic generalization of amoebas leans to the less sexy side, which is almost certainly wrong, concludes a study published online March 23 in Proceedings of the Royal Society B (for which Spiegel was a reviewer and on which he based his commentary). Although some amoebas haven't been caught having sex, the authors' taxonomic work suggests amoebic ancestors did do it—just like the common ancestor that led to all modern eukaryotes.

It's not certain what pushed some amoebas into celibacy, but they may have evolved in stable environments that didn't require the energy costs of, or genetic advantages conferred by sex. After dropping the ability to sexually shuffle their genes, perhaps they simply got by reproduction via mitosis. Whatever the case may be, it's fertile ground for more research.

"Sex is an expensive process associated with big changes in an environment," Spiegel says. "I like to tell my students, 'When the going gets tough, the tough get horny.'"

In addition to dealing with asexuality, early taxonomists had to sort amoebas that resembled plants, fungi and animals. When Robert Whittaker debuted his popular five-kingdom classification system 42 years ago, he plucked amoebas out of various kingdoms and deemed them all protists. Genomic research and the field of systematic biology have since refined this sorting, but Spiegel argues outdated ideas continue to distort evolutionary relationships among organisms.

He isn't alone in his desire to restore amoebas to their proper sexual place or to grumble about misconceptions of biological sex, including the notion that it's reproductive. (Sex is not reproductive, he says, but rather a reduction of two gamete cells into one—it's mitosis, the duplication of cells, that is reproductive.*)

"We don't completely understand the evolutionary processes behind sex, or even its benefits," says taxonomist David J. Patterson of the Marine Biological Laboratory, who was neither involved in Spiegel's piece nor the study of amoeba asexuality from which it stemmed. "What we need to do is run back down [the evolutionary] tree and study sex from the bottom up, looking for alternative options to sex."

As some biology textbooks and studies continue to sidestep the details of the amoeba and other microbes in favor of focusing on larger organisms, Spiegel sees a tragic irony: Amoebas and their ilk can best position budding researchers to explore the costs of sex, its evolution and alternatives to problems it may or may not solve.

"They are using the wrong organisms to ask the questions [about sex] they want to answer,” Spiegel says.

Biologist John Bonner of Princeton University, who has studied amoebas for more than 70 years, says he has no reason to doubt Spiegel's concerns—even if they are not new.

"Sex is quite common among [amoebas], even though some are asexual. And I think there are probably a lot of people who still don't understand that," Bonner says. "It's definitely worth calling attention to. There are still some very important things to be learned by comparing the presence and absence of sex."